1. Field of the Invention
The present invention relates to wireless communication. In particular, the present invention relates to communication between a mobile station (MS) and elements of a wireless network through one or more relay stations (RSs).
2. Discussion of the Related Art
Various cooperative transmission schemes are known to use virtual antenna systems to achieve cooperative diversity. Most of such cooperative transmission systems, however, focus on a single relay or ignore the latency caused by the unreliability between base stations (BSs) and the RSs.
One cooperative transmission scheme is disclosed in the article “Distributed space-time-coded protocols for exploiting cooperative diversity in wireless networks,” by J. N. Laneman and G. W. Wornell, published in IEEE Trans. Inf. Theory, vol. 49, no. 10, pp. 2415-2425, October 2003 (“Laneman I”). In Laneman I, space-time coded cooperative diversity protocols are developed for combating multipath fading across multiple protocol layers in a wireless network. In that article, the authors prove theoretically that a space-time coded cooperative diversity scheme can provide full spatial diversity. However, the article does not show clearly how the protocols may be implemented in practice.
Another cooperative transmission scheme is disclosed in the article “Cooperative diversity in wireless networks: Efficient protocols and outage behavior,” by J. N. Laneman, D. N. C. Tse, and G. W. Womell, published in IEEE Trans. Inf. Theory, vol. 51, no. 12, pp. 3062-3080, December 2004 (“Laneman II”). In Laneman II, the authors disclose efficient protocols to achieve cooperative diversity. There, several strategies employed by the cooperating radios are outlined, including fixed relaying schemes (e.g., amplify-and-forward and decode-and-forward schemes, relay selection schemes that adapt based upon channel measurements between the cooperating terminals, and incremental relaying schemes that adapt based upon limited feedback from the destination terminal). To improve spectral efficiency, a hybrid automatic-repeat-request (HARQ) is introduced between the BS and the MS. However, Laneman II does not address the possibility of a long delay that may result when the data received at the RSs are not reliable.
A different approach is discussed in the article “Practical relay networks: A generalization of hybrid-ARQ,” by B. Zhao and M. C. Valenti, published in IEEE J. Sel. Areas Commun., vol. 23, no. 1, pp. 7-18, January 2005 (“Zhao”). Zhao discloses multiple relays operating over orthogonal time slots, based on a generalization of HARQ. Unlike conventional HARQ, retransmitted packets need not originate from the original source; rather, retransmission packets may be sent by RSs that overhear the transmission. Similarly, other HARQ-based schemes are discussed in the articles (a) “Achievable diversity-multiplexing-delay tradeoff in half-duplex ARQ relay channels”, by T. Tabet, S. Dusad and R. Knopp, published in Proc. IEEE Int. Sym. On Inf. Theory, pp. 1828-1832, September 2005 (“Tabet”); and (b) “Hybrid-ARQ in multihop networks with opportunistic relay selection” (“Lo”), by C. K. Lo, R. W. Heath, Jr. and S. Vishwanath, published in Proc. IEEE Int. Conf. on Acoustics, Speech, and Signal Proc., pp. 617-620, April, 2007. Tabet and Lo both discuss HARQ-based schemes for delay-limited fading single relay channel. However, Zhao, Tabet and Lo are all limited to a single RS. Cooperation among several RSs is not exploited.
U.S. Patent Application Publication 2006/0239222 A1, entitled “Method of providing cooperative diversity in a MIMO wireless network,” by S. Kim and H. Kim (“Kim”), discloses a method for providing cooperative diversity in a multiple-input-multiple-output (MIMO) wireless network. In Kim, the RSs check for errors, relay the verified streams (i.e., “correct” streams) and ask for the retransmission of error streams from the BS. However, Kim's method investigates only a single RS and does not consider cooperative diversity.
Three cooperative relay schemes are discussed in the articles (a) “Cooperative relay approaches in IEEE 802.16j,” W. Ni, L. Cai, G. Shen and S. Jin, published as IEEE C80216j-07/258, March 2007 (“Ni”) and (b) “Clarifications on cooperative relaying,” by J. Chui, A. Chindapol, K. Lee, Y. Kim, C. Kim, B. Kwak, S. Chang, D. H. Ahn, Y. Kim, C. Lee, B. Yoo, P. Wang, A. Boariu, S. Maheshwari and Y. Saifullah, published in IEEE C80216j-07/242r2, March 2007 (“Chui”). The cooperative relay schemes are (i) cooperative source diversity, (ii) cooperative transmit diversity and (iii) cooperative hybrid diversity. However, none of these cooperative relay schemes consider the reliability of the channels between BS and RSs. When the information regenerated by the RSs is not reliable, the effects of cooperative diversity may deteriorate, thereby degrading the overall performance at the MSs.
The article “An ARQ in 802.16j,” by S. Jin, C. Yoon, Y. Kim, B. Kwak, K. Lee, A. Chindapol and Y. Saifullah, published in IEEE C802.16j-07/250r4, March 2007 (“Jin”) discloses a cooperative scheme in which ARQ provides fast recovery of the source data. In Jin, the BS retransmits source information when any RS fails to receive a block from the BS. Jin's scheme reduces latency by retransmitting only between RSs and the BS. However, when multiple RSs are present, a substantial delay may result if the BS has to retransmit until all RSs successfully receive the source block. The retransmission also reduces spectral efficiency. Jin's model can be improved as it is not necessary for all RSs to receive the correct source block. As long as a significant number of RSs receive the source block reliably, the other unreliable RSs may obtain the correct source block by overhearing the transmissions between the reliable RSs and the MSs.
While most cooperative schemes (e.g., Zhao, Tabet and Lo) assume only one RS is involved in the retransmission at each hop, such schemes fail to realize that multiple RSs can provide higher cooperative diversity. Also, the reliability of the channel between BS and RSs are not considered in some cooperative schemes, such as Laneman I and II, Ni and Chui. Unreliable information at the RSs may degrade MS performance and may cause extensive delays.
Retransmission by the BS also reduces spectral efficiency of the system. As discussed in Jin, a BS has to retransmit the data even if only one RS does not obtain the reliable data. Such a scheme introduces latency and may result in a deadlock between the BS and RSs when the number of RSs increases.
Except for Zhao, all the above schemes ignore the radio resources among RSs (i.e., the RS which does not receive the reliable information from the BS may be able to overhear the transmission between the reliable RSs and the MSs). Zhao, however, focuses on selecting one RS at each hop.